Cook spinach according to package directions; drain and press dry. Set aside.
Sauté onion in butter in a heavy saucepan over low heat until tender. Blend in flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually stir in milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and pepper.
Beat egg yolks. Add one-fourth of hot white sauce to yolks, stirring well; return yolk mixture to white sauce. Add spinach and cheese; stir well.
Beat egg whites (at room temperature) until stiff but not dry. Fold into spinach mixture. Spoon into a well-greased 2- quart casserole. Bake at 350° for 45 minutes or until golden brown.